Her husband, Richard Ratcliffe, who has long campaigned for her release, said she been refused medical attention for lumps in her breasts, neurological care for pains in her limbs and access to an external psychiatrist.

The first wave of her strike – alongside Iranian human rights defender Narges Mohammadi, who is also detained in Evin – was to last three days.

But she will consider extending it if her demands to see a doctor are not met.

Redress argues that her release should be secured by Britain granting her diplomatic protection, a process under international law that states can enact to obtain repatriation for an illegal act against one of their nationals.

A Foreign Office spokeswoman said: “The Foreign Secretary has discussed this issue with Richard Ratcliffe, and is keen to take a decision as soon as possible.

“We continue to take action on all our consular cases in Iran in line with what we believe will produce the best outcomes in their cases.”

Thomson Reuters Foundation chief executive Monique Villa has said it is “extremely shocking” to see a “totally innocent” employee begin such a drastic protest.